Research is under way to develop control options to delay or minimise the development of glyphosate-resistant ryegrass.

Western Australia Department of Agriculture research officer Abul Hashem said the research project was trialling management options such as integrated weed management and herbicide rotation.

"Carried out in collaboration with the Western Australian Herbicide Resistance Initiative at University of Western Australia, the project has demonstrated that glyphosate resistance in ryegrass is on the increase in Australia," he said.

"Accordingly, we are now looking at alternative weed control options, including the development of a double knockdown sequence, and using novel herbicide mixtures involving herbicides with varied modes of action."

Dr Hashem said glyphosate continued to be the single most important knockdown herbicide in controlling ryegrass at pre-seeding.

"However, the evolution of a resistant ryegrass biotype is likely to occur should there continue to be a high reliance on knockdown herbicides, with repeated application of the same mode of action."

Dr Hashem said evidence of resistance had been discovered by assessing the effectiveness of glyphosate application rates on three generations of resistant ryegrass populations over three years.

"As part of the Grains Research & Development Corporation (GRDC) funded project, seeds and plants were collected from 52 populations where there was an unexplained failure in the control of ryegrass," he said.

"Two populations showed resistance to glyphosate in the initial test, and these were grown over three years and tested for resistance against different rates of glyphosate."

Dr Hashem said resistance was detected in all populations when applied at a rate of 0.5 kg ae/ha and one kg ae/ha of glyphosate (equivalent to two L/ha roundup®).

When applied at a rate of two kg ae/ha of glyphosate, seven per cent of plants from one of the populations survived.

"This finding is of concern given that the recommended rate of glyphosate in Western Australia for annual ryegrass under minimum tillage is only 0.54 to 0.72 kg ae/ha," he said.